This invention broadly relates to a new UV curable pressure sensitive adhesive composition and method of using same. By the term "UV curable" as used herein it is meant curable with ultraviolet radiation.
Pressure sensitive adhesives are materials that form a bond with very light pressure. The cured adhesive is neither solid nor liquid, but rather is a viscoelastic material that is usually quite tacky. While pressure sensitive adhesives are available as 100% solids for hot melt applications, and as water-based emulsion acrylic systems, the majority of the approximately 500 million dry pound domestic market in the U.S.A. are solvent based products. UV curable pressure sensitive adhesives represent only a few percent of the market, in large part because of the problem with their historical poor high temperature properties. In the past it has been thought that a UV curable pressure sensitive adhesive exhibiting acceptable high temperature and environmental performance characteristics could capture a large percent of the market share for pressure sensitive adhesives. Also in the past, there have been some UV curable pressure sensitive adhesives which included a polyurethane resin; however, they did not have the necessary integrity or performance characteristics at temperatures above approximately 130.degree.-140.degree. F., and they also exhibited poor humidity resistance.
While UV curable pressure sensitive adhesives do not currently command a significant market share, the interest remains high in this type of pressure sensitive adhesive technology. Various other companies and individuals have worked on solving the problems associated with this technology; however, such other parties have not been able to develop satisfactory products which would be usable and worthwhile in industry.
Pressure sensitive adhesives (PSAs) are tacky substances normally or often placed on backing materials which, when brought in contact with a surface under light contact exhibit sufficient cohesiveness as to resist peeling away from that surface. Classic examples of the usage of pressure sensitive adhesives are tapes, bandaids, and labels.
The chemistry of pressure sensitive adhesives can be roughly sub-divided into three large groups: 1) solvent-based adhesives, 2) water-based adhesives, and 3) radiation cured adhesives.
Solvent-based adhesives are typically ultra-high molecular weight (Mw), soft polymers (low Tg) dissolved in solvent; when applied to a backing material, they provide tack or stickiness of the backing material to a contacted surface. While the chemistry involved is complex, involving different chemical polymers and block copolymer polymerizations, it can be reduced to one generic classification--thermoplastic polymers (TPs). Thermoplastic polymers on this theme are large, low Tg molecules which entangle on a molecular level, giving tack and strength at room temperature. (Tg means glass transition temperature). The liability of TPs is high-temperature strength. Typically at temperatures exceeding 120.degree. F., all strength is lost as the polymers adopt a parallel arrangement to force lines when stressed. Higher Mw polymers, in general, behave better at elevated temperatures, but usually require higher VOC solutions, and still the integrity at temperatures of approximately 140.degree. F. or higher is suspect. VOC stands for volatile organic compound(s).
Water-based PSAs can be considered as nothing more than variants on the thermoplastic polymer theme. Again, low Tg, ultra-high Mw polymers are suspended in solution (water); film formation and chain entanglement then occur when the latex particles coalesce with drying.
The advantages of water-based PSAs are the abatement of air pollution (low VOC) and generally enhanced oxidation resistance of the final film. The liability of this chemistry is identical to its solvent-based counterpart;--high-temperature strength. To achieve better strength at high temperatures, higher Tg polymers are sometimes used and the trade-off is always tack;--the end-product does not efficiently grab contacting surfaces. Tack and high-temperature strength are the features which are diametrically opposed in TP chemistry.
Radiation-cured PSAs can be subdivided into electron beam (EB) films and UV-curable (UV) films. The chemistry is similar in that uncured mixtures of unsaturated polymerizable compounds are irradiated in both instances, but EB films have dominated the past high-temperature market. Electron beam adhesives are usually formed by applying solutions (VOCs) of high Mw polymers with a small amount of double bonds to the substrate they will be used on, and then exposing the resultant film to a high-intensity electron gun. The electrons secondarily crosslink the already high Mw material giving a thermoset nature to the end-product. Final film Tg is not dramatically enhanced; Mw of the polymers approaches infinity, and therefore, the result is a film which has strength at elevated temperatures, up to 200.degree. F., and tack at room temperature. EB films are satisfactory for some applications, but solutions must be used to apply the initially high MW prepolymers (e.g., detrimental VOC's), and the electron beam process is slow cumbersome, and costly. An EB gun costs approximately one million dollars.
UV cure is attractive, therefore, because it involves 100% convertible material with a simpler and more economical manufacturing process. Up to this point combinations of Mw oligomers have had to be used for application purposes; high Mw prepolymers are much higher in viscosity and, therefore, require a solvent. The problem in using low Mw oligomers is that it has been virtually impossible to generate high Mw end-products in the curing time frames usually encountered. The result is a lower overall Mw film which obtains its integrity via the incomplete polymerization of higher Tg starting materials. While this can give decent room temperature tack and cohesion, high-temperature strength is severely compromised. In the past UV-curable PSAs have had virtually no strength at 130.degree. F.
Accordingly it is a primary object of the present invention to provide a new UV curable pressure sensitive adhesive composition, and method of using same.
Another object of the present invention is to provide a new UV curable pressure sensitive adhesive composition which exhibits good performance characteristics at temperatures in the range of approximately 130.degree.-140.degree. F. or higher.
Another object of the present invention is to provide a new UV curable pressure sensitive adhesive composition which exhibits excellent properties in terms of humidity resistance and high temperature performance.
Another object of the present invention is to provide a novel pressure sensitive adhesive composition which is environmentally friendly and economical to produce.
Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims.